The present invention relates to a measuring conduit which is used to simultaneously measure the flow rate of a fluid such as a respiratory gas and the concentration of a particular component in the fluid.
In the case of a fluid containing various kinds of components, there is often a demand for the measurement of the flow rate of a particular component. For example, when air or gas in which oxygen is artifically added to the air, such as a respiratory gas, is inhaled and the gas containing carbon dioxide is exhaled, the flow rate of carbon dioxide in the expiratory gas is used as carbon dioxide production for the evaluation of a respiration function.
A method which has been conventionally performed to measure carbon dioxide production is called the Douglas bag method. According to this method, an expired gas is collected into an airtight bag, and the product of its volume v and the carbon dioxide concentration F.sub.CO2 is obtained. Finally, carbon dioxide production is determined by the average value during measuring period.
Due to the recent advancement in methods for measuring the flow rate of a fluid and for analyzing a component, the real-time measurement of the flow rate of the component has been made possible. For instance, to measure carbon dioxide production V.sub.CO2 (t), the instantaneous flow rate v(t) of expired gas and the instantaneous concentration of carbon dioxide F.sub.CO2 (t) are simultaneously measured, and carbon dioxide production V.sub.CO2 (t) is computed in the form of: EQU V.sub.CO2 (t)=v(t).multidot.F.sub.CO2 (t).
The flow-rate measurement and the concentration measurement are each performed while a fluid is flowing through a measuring conduit. To measure the flow rate and the concentration at a time, therefore, a measuring conduit for measuring the flow rate and a measuring conduit for measuring the concentration must be coupled in tandem. However, such a coupling of two measuring conduits makes the dead space in the respiration circuit fairly large. The resultant dead space equals the sum of the capacities of the measuring conduits. This puts a great burden on the respiratory organ of an examinee, causing difficulty in the examination of a serious patient.
In addition, the tandem coupling of the measuring conduits also causes a problem of measurement accuracy. A reason for such a problem is that both measuring times are slightly different due to the distance between two measuring points. Namely, assuming that the distance from the flow-rate measuring point to the concentration measuring point is L and a flow velocity of the fluid is V, the time t.sub.d (time difference) required for a lump of fluid undergoing the flow-rate measurement to reach the concentration measuring point is given by: EQU t.sub.d =L/V.
From this it is evident that the time t.sub.d changes depending on the flow velocity. In order to eliminate the influence of this time difference t.sub.d, the carbon dioxide production has been conventionally found as follows: EQU V.sub.CO2 (t)=v(t).multidot.F.sub.CO2 (t-t.sub.d).
However, particularly in the case of a compressive fluid such as a respiratory gas, the flow velocity variously changes in one respiration, and a change in flow velocity is also caused due to a change in the pressure of such fluid. Therefore, the time difference t.sub.d complicatedly varies, and it becomes extremely difficult to strictly perform the correction as shown in the above equation.